The security breaches happened in January and February.
Chinese government hackers compromised the computers of a U.S. Navy contractor and stole a large amount of highly sensitive data on undersea warfare, the Washington Post reported on Friday, citing unnamed U.S. officials.
The Post said that those behind the breach are alleged to have obtained secret plans for a supersonic anti-ship missile.
As a whole, the stolen data can be considered as classified, according to the officials, even though the data was being stored on the contractor's unclassified network. Bill Speaks told the DC paper.
According to the Post, hackers swiped 614 gigabytes of data that included information relating to sensors, submarine cryptographic systems and a little-known project called Sea Dragon.
Following the Post's report, Defense Secretary James Mattis directed the Pentagon's Inspector General to investigate the incident.
'Per federal regulations, there are measures in place that require companies to notify the government when a "cyber incident" has occurred that has actual or potential adverse effects on their networks that contain controlled unclassified information, ' the U.S. Navy said in response to a query from Reuters.
The hackers targeted the contractor who works for the Naval Undersea Warfare Centre - a military organisation headquartered in Rhode Island's Newport city - that conducts research and development for submarines and underwater weaponry, The Washington Post reported.
The Washington Post said it agreed to withhold some details at the request of the United States navy over fears it could compromise national security. The White House is seeking Beijing's help in negotiating with North Korea over its nuclear weapons, while at the same time sparring with China over trade and China's territorial claims in the South China Sea, reports the Hill.
The breach continues China's longstanding effort to "blunt the US advantage in military technology and become the pre-eminent power in east Asia", the Post reported.
The Pentagon has spent more than $300 million since 2015 on the Sea Dragon project, which involves adapting current Navy technology for new uses.